1. Field of the Invention
The present invention relates to a thin-film capacitor element for use in small electronic circuits and an electronic circuit board over which, together with this thin-film capacitor element, thin-film circuit elements, such as inductor elements, are to be formed.
2. Description of the Prior Art
In recent years, along with the advancement of integrated circuit technology, downsizing of electronic circuits is further proceeding, and there have been developed small electronic circuit boards over which capacitors, resistors, inductors and the like are to be formed as thin films.
Substrates for such electronic circuit boards can be made of a monocrystalline material such as sapphire or a sintered material such as alumina. As alumina is relatively inexpensive and excels in high frequency performance among these available materials, high frequency devices extensively use alumina substrates. Over an alumina substrate, various required circuit elements are formed as a set of thin films (hereinafter referred to as being formed in a filmy state). For instance, a capacitor element is configured over an alumina substrate by successively stacking layers of a lower electrode, a dielectric layer and an upper electrode. A resistor element is configured by forming in a filmy state a resistance layer of a desired shape over an alumina substrate and forming, also in a filmy state, electrodes at its two ends. An inductor element is configured by forming in a filmy state a metallic film of a desired shape over an alumina substrate and forming, also in a filmy state, electrodes at its two ends. Further over the alumina substrate path, a transmission line is formed as an electroconductive pattern, and this transmission line is connected to the respective electrodes of the capacitor element, resistor element and inductor element.
FIG. 12 is a plan of a conventionally known thin-film capacitor element, and FIG. 13, a sectional view along line 13xe2x80x9413 in FIG. 12. As these drawings show, a conventional thin-film capacitor element consists of a stacked structure of a lower electrode 31, a dielectric layer 32 and an upper electrode 33 formed over a substrate 30, and the capacitance of the capacitor is determined by the range in which the lower electrode 31 and the upper electrode 33 overlap each other.
The lower electrode 31 is formed of Cu or the like sputtered or plated over the substrate 30 and etched into a desired pattern shape. The dielectric layer 32 is formed of SiO2 or the like sputtered or chemical vapor-deposited over the lower electrode 31 and the substrate 30 and etched into a desired pattern shape. The patterned dielectric layer 32 extends to over the substrate 30 via the top face and side faces of the lower electrode 31. The upper electrode 33 is formed of Cu or the like sputtered or plated over the dielectric layer 32 and the substrate 30 and etched into a desired pattern shape. The patterned upper electrode 33 extends to over the substrate 30 via the top face and side faces of the dielectric layer 32.
Incidentally, one of the requirements for such a thin-film capacitor element concerns the breakdown voltage between the lower electrode 31 and the upper electrode 33. If this breakdown voltage is below the required level, the thin-film capacitor element will be broken down at a low voltage and can no longer operate as such. The breakdown voltage is heavily dependent on the thickness of the dielectric layer 32 intervening between the two electrodes 31 and 33. Increasing the thickness of the dielectric layer 32 would raise the breakdown voltage, but in the conventional thin-film capacitor element described above the dielectric layer 32 becomes thinner near the corners of the lower electrode 31 (see section P in FIG. 13) and thereby invites a drop of the breakdown voltage. This is due to the circumstance that when the dielectric layer 32 is formed over a level gap resulting from the etching of the lower electrode 31, the coverage of the dielectric layer 32 is adversely affected near the corners of the lower electrode 31. Especially where the dielectric layer 32 is formed by sputtering, it is difficult for sputtered atoms to adhere to the vertical face of the substrate 30 with the consequence that a drop or fluctuations of the breakdown voltage become conspicuous.
Although the dielectric layer 32 can be improved by thinning the lower electrode 31 to reduce the level gap, the thinner the lower electrode 31 and the upper electrode 33, the greater the resistance component of conductors in series, which would give rise to another problem of an lowered Q value of the thin-film capacitor element. Or if the overall thickness of the dielectric layer 32 is increased, the breakdown voltage can be prevented from dropping, but the thicker the dielectric layer 32, the smaller its capacitance per unit square measure. Accordingly, the thin-film capacitor element will become bigger and, moreover, as a greater thickness of the dielectric layer 32 does not serve to stabilize the shape of coverage, fluctuations of the breakdown voltage will remain.
In the conventional thin-film capacitor element described above, the capacitance of the capacitor is determined by the range in which the lower electrode 31 and the upper electrode 33 overlap each other (the rectangle of Bxc3x97C in FIG. 12). When the lower electrode 31 and the upper electrode 33 are etched into a desired pattern shape, the size and alignment of the two electrodes 31 and 33 tend to be adversely affected in accuracy by fluctuations in side etching quantity and etch rate, giving rise to a problem of fluctuations in the capacitance of the capacitor element. This problem is more serious with thin-film capacitor elements of smaller capacitances, because a smaller capacitance means a smaller area of overlap between the lower electrode 31 and the upper electrode 33 and accordingly the impact of accuracy fluctuations of etching and overlapping on the capacitance increases in relative terms.
Although an alumina substrate used in this kind of electronic circuit board has the aforementioned advantages, at the same time it is inferior in surface smoothness to sapphire and other monocrystalline substrates. The surface roughness (Ra) of an alumina substrate of 99.5% in purity, for instance, is 30 to 100 nm in terms of unevenness. As a consequence, when a capacitor element film is formed over an alumina substrate whose surface smoothness is so poor, the thickness of the dielectric layer formed over the lower electrode becomes partly thin, resulting in a problem of a significantly lowered breakdown voltage.
In view of this problem, in order to smoothen the alumina substrate surface, there are known such methods as mirror-grinding the whole surface of the alumina substrate or coating it with an insulating film of a polymer material or glass. However, the grinding method involves the disadvantages of leaving small dents between crystals in the alumina substrate and moreover, as an alumina substrate is hard, taking a long time. On the other hand, coating with an insulating film creates no big problem to the capacitor element, which is a capacitive one among the various filmy circuit elements and transmission lines formed over the insulating film, because the dielectric loss of the insulating film of a polymer or glass, it does, however, invite increased dielectric losses in other resistance elements, inductor element or layers under the transmission lines, which might lead to a deterioration the high frequency performance of the high frequency device.
In a thin-film capacitor element according to the present invention, a lower electrode and a dielectric layer are successively stacked over a substrate, the periphery of this dielectric layer is covered with an insulating layer having an opening, and an upper electrode formed over this insulating layer is stacked over the dielectric layer within the opening.
This configuration, which ensures insulation between the lower electrode and the upper electrode by the insulating layer covering the dielectric layer, can securely prevent any drop or fluctuation of the breakdown voltage due to inadequate coverage of the dielectric layer. Furthermore, as the opening in the insulating layer determines the capacitance of the capacitor, fluctuations in capacitance can be reduced irrespective of the accuracy of the lower electrode and the upper electrode in size and alignment.
In the above-described configuration, it is preferable to form the insulating layer of a photosensitive resist. The formation of the insulating layer of a photosensitive resist makes possible highly accurate formation of the opening and more effective reduction of capacitance fluctuations.
In the above-described configuration, though it is sufficient to form the dielectric layer at least over the surface of the lower electrode, it is possible to form the dielectric layer continuously from the top and side faces of the lower electrode to the substrate, and in this case, as the lower electrode and the upper electrode are insulated by two layers, i.e. the dielectric layer and the insulating layer, even if the coverage of the dielectric layer is inadequate in the level gap of the lower electrode, any drop or fluctuation of the breakdown voltage can be securely prevented in spite of this inadequate coverage.
Alternatively, it is also possible to form the dielectric layer only over the surface of the lower electrode and part of the insulating layer from the periphery of the dielectric layer to over the substrate. Although in this case insulation between the lower electrode and the upper electrode is accomplished by the insulating layer alone, the formation of part of the insulating layer directly over the substrate can enhance the tightness of the adhesion of the insulating layer.
In the electronic circuit board according to the invention, an insulating layer is formed over part of an alumina substrate, and a capacitor element consisting of a lower electrode, a dielectric layer and an upper electrode stacked successively is formed over this insulating layer, and at least an inductor element and a transmission line are formed, each in a filmy state, over the alumina substrate.
In the electronic circuit board configured in this manner, as fine ups and downs on the surface of the alumina substrate are evened out by the insulating layer, the breakdown voltage of the capacitor element formed in a filmy state over the insulating can be prevented from dropping. Furthermore, as the insulating film which suffers a great dielectric loss does not adversely affect the inductor element and the transmission line, the high frequency performance can be prevented from deteriorating.
In the above-described configuration, it is preferable for the lower electrode and upper electrode of the capacitor element to overlap each other within a boundary defined by (hereinafter referred to as the contour of) the insulating layer, and this disposition can securely prevent the breakdown voltage of the capacitor element from dropping.
Further in the above-described configuration, although it is also possible to pattern glass film coating over the whole alumina substrate surface in a desired shape as a means to partly form an insulating layer over the alumina substrate, it is more preferable to form the insulating layer by exposing to light and developing a positive type photosensitive polymer film, such as a photoresist. In this way, an insulating layer of any desired shape can be obtained more simply and yet more accurately.
Where the circuit elements in the foregoing configuration are to include a resistance element besides a capacitor element and an inductor element, especially where a material having a relatively high specific resistance, such as TaSiO2, is used as the resistance layer of the resistance element, it is preferable from the viewpoint of preventing deterioration of the high frequency performance to form in a filmy state this resistance layer over the surface of the alumina substrate like the inductor element and the transmission line. On the other hand, where a material having a relatively low specific resistance, such as Ta2N, is used as the resistance layer of the resistance element, it is preferable to form in a filmy state this resistance layer over the insulating layer like capacitor element. In this way, although the resistance layer underneath the resistance element somewhat deteriorates the high frequency performance, the resistance of the resistance element formed in a filmy state over the insulating layer can be prevented from fluctuating substantially, because fine ups and downs of the alumina substrate are smoothened by the insulating layer.